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Full text of "Handbook Of Chemical Engineering - I"

THE TRANSPORTATION OF GASES
159
It is not practical to carry the expansion piece on a fan outlet to a point where the velocity is below 1,000 to 500 ft. per minute. The velocity head due to a velocity of 500 ft. per minute, is but ^4 in.., and is negligible. The angle between the sides and the axis of an efficient chimney or diffuser should be from 7 deg. to 10 deg. The use of a diifuser produces a lower static pressure at the cutoff point and consequently the fan may run at a lower speed and at higher volumetric efficiency. Actual tests on a multiblade fan for mine ventilation
showed that, when fitted with a chimney whose larger     ^   SQQ ^ JSOQ ^ 2Mo 3ooo end was twice the cutoff area and whose sides had a            volume,cubicnserper Minute."
taper of 8 deg. from the axis, the. volumetric capacity  FlG- i4;—Effeot of using an
^   .              i ., rt               j.      i                  j.-             •    j.            evas6 discharge on fan.
was increased 12 per cent when operating against a
given static pressure and at a given number of revolutions per minute.   The horsepower, however, increased only 5 per cent.
Diffuser Casing Open Around Circumference.—Similar results may be obtained with wheels discharging into circular diffusers open all around their periphery. Figure 10 shows this form of diifuser, which incidentally has still another
100   WO  '300  400    500   600   700   800 Volume, Cubic Feet per Minute.
FIG. 15.—Characteristics of a single-inlet steel-plate paddle-wheel fan with radial blades.
600     600     1000     IZOO Volume,Cubic Teet per Minute.
FIG. 16.—Characteristics of a single-inlet fan for high speeds and low pressures.
effect on the outflow of air. This type of fan must deliver its air against the pressure in the chamber into which it discharges, and this pressure acts in a radial direction at all points. It follows, therefore, that the resultant direction of flow should be as near radial as possible. This necessitates a blade curved backward relative to rotation, but it can be considerably helped by a radial diffuser. Figure 11 shows that the velocity of the air decreases in this diffuser and at the same time the direction of the flow becomes more nearly radial; also, that the radial component of the air velocity becomes greater relative to the resultant as the distance from the center increases, thus utilizing a greater part of the resultant velocity. In Fig. 11, r2 is the radius of the wheel or entrance to diffuser, r3 the outer radius of diffuser, and A 2 is the circumferential area at entrance to the diffuser. The volume delivered is ti2A 2 = v^smbzAi. No force acts on the air as it passes through the diffuser and its direction is unchanged, but its radial component varies inversely as the radius, or U3/uz = r2/r3. The direction of the air leaving the diffuser is known, and its velocity is determinable since w3 is known. Both v* and w$ decrease much more rapidly than inversely as the radius, and the ratio of